The present invention evolved from continuing development efforts pertaining to subject matter disclosed in my co-pending Application Ser. No. 216,872, filed Dec. 15, 1980, entitled "Balun Coupled Microwave Frequency Converter" now U.S. Pat. No. 4,330,868. This co-pending Application discloses a diode mixer bridge with a port, such as the RF input port, supplied by balanced coplanar conductors.
The present invention relates to improvements in the microstrip circuit layout and structure for ports to a mixer bridge. The invention enables symmetry among the microwave conductors leading to the bridge in a compact layout. This compact symmetry of the mixer port structure significantly reduces unwanted sensitivity to adverse parameters, and also overcomes previous problems of distortion caused by unequal conduction paths and diode lead lengths.
Transmission lengths along the microwave conductors leading to the mixer bridge are minimized, to reduce loss and exposure to mismatch influences. Transmission path lengths through the diodes of the mixer bridge are also minimized, and are substantially equal, to thus eliminate unwanted phase shift due to differential path lengths through the mixer diodes. The diode mixer leads are short and orthogonal, to provide good impedance matching and minimize unwanted coupling. The microwave conductors supplying the ports to the mixer are also orthogonal, to reduce unwanted coupling therebetween.
In another desirable aspect of the invention, the substrate circuit layout affords keyed alignment mounting of the diode bridge in a singular precise position and positively locks the bridge in place. This positive, singular registry of the diode bridge affords consistency of mounting location and orientation, which eliminates circuit unbalance and differing performance from one unit to another otherwise due to imprecise variable positioning of the bridge. Even the slightest offset or lateral displacement can cause different path lengths and thus unwanted phase shifts.
In a further desirable aspect of the invention, a single mixer of the diode-quad bridge type is provided and affords microwave frequency conversion without the undesirable characteristics of slot line transmission. Image frequency signals generated in the two diode pairs circulate in the diode quad only, to return the image frequency power to the diodes to reduce conversion loss. There is mutual isolation between RF, LO and IF signals over a broad frequency range. The circuit accepts transposed RF and LO inputs.
It has been found that the operating bandwidth of the mixer is significantly increased by the use of the balanced port structure of the invention, the equalized conduction path lengths, the short lead lengths, and single point connections between the diode pairs facilitating "zero lead length" between the diode connection points.
In another aspect of the invention, standard printed circuit board techniques may be used for fabrication, resulting in significant cost reduction. The invention also enables the use of low dielectric constant substrate material, further reducing cost.